Jeremy Corbyn has strongly defended the Stop the War coalition and declared his complete backing for the organisation, ignoring repeated calls from his internal Labour opponents to cut ties with the organisation.

The Labour leader issued a defiant statement in support of Stop the War, calling it "one of the most important democratic campaigns of modern times", despite complaints from some Labour MPs regarding the organisation's stance on Syria and the deletion of certain posts on Stop the War's website.

Corbyn served as chair of the organisation until earlier this year and was greeted by a standing ovation when he turned up at the anti-war group's £50-a-ticket Christmas party in central London on Friday night. The Labour leader entered the restaurant via a back entrance to avoid a demonstration and waiting journalists.

"[Stop the War] has brought hundreds of thousands of people on to the streets time and again," Corbyn told the audience at the event. "It has organised protests and lobbies in every part of the country, including by military families. Most of all, it has been shown to be right in opposing more than a decade of disastrous wars – in Afghanistan, Iraq, and Libya – while many of its most vociferous critics supported them.

"The anti-war movement has been a vital force at the heart of our democracy. Branding it as somehow illegitimate is an attempt to close down democratic debate and campaigning."

But outside the event – where other entertainment included a speech by Brian Eno and a performance by a musician described as a "busker with a yellow violin" –opponents of the group, many of them Labour supporters, called on Corbyn to change his mind.

Abdulaziz Almashi, co-founder of the Syria solidarity movement that opposes President Bashar al-Assad, said Corbyn's comments were hypocritical because STW is not opposing the Syrian leader: "If Jeremy Corbyn believes Stop the War is one of most democratic movements in this country then why don't Stop the War support the democratic movement against the dictator Assad, who came to power illegally?"

Almashi is a constituent of Corbyn and was quoted approvingly by the Labour leader during his House of Commons speech against bombing ISIS targets in Syria. But he told BuzzFeed News that Stop the War would not listen to his movement because they were anti-Assad: "I am a Syrian. I am against bombing of my country. Jeremy Corbyn I believe and respect him but Stop the War coalition are wrong when it comes to Syrian crisis.

"They don't recognise the Syrian people have started a peaceful revolution. Why has Stop the War never organised a demo against the Syrian regime? Why don't they organise a demo outside the Russian embassy against their intervention?

"If they are anti-intervention they cannot be selective in their anti-intervention," he added.

Veteran campaigner Peter Tatchell was also protesting outside the venue and said he had quit the organisation over its stance on Syria: "We think it's very important to hold Stop the War to account for its misguided polices on Syria. I don't support bombing Syria but I strongly object to the way Stop the War has never organised or supported any protests against Assad's war crimes or against the military interventions by Russia, Iran, and Hezbollah.

"It's a one-sided selective anti-war movement. Equally outrageous, STW has repeatedly refused to allowed Syrian democrats and left-wingers to speak at its meetings."

He concluded: "I have supported STW from the outset of its formation in 2001. It was absolutely right to oppose wars in Afghan and Iraq. But it's got things badly wrong on Syria."

Jim Waterson is a politics editor for BuzzFeed News and is based in London.